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I overzealously landed https://crrev.com/c/990512 assuming that if a
TCP connection is open through an HTTP/2 proxy with an empty ALPN list,
then the server cannot negotiate HTTP/2.  Turns out that there are
already 11 crashes reported on the first day of the Canary release
that picked up this change.  This CL adds a unittest that triggers this
CHECK (verified locally), and changes Job::DoInitConnectionComplete() to
handle this case properly and avoid the crash.

Bug: 828865
Change-Id: I5bc8fb447a07d350970904b350f9aeb670e2da7d
Reviewed-on: https://chromium-review.googlesource.com/996878Reviewed-by: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548520}